When nondestructively testing solid materials for defects by ultrasonic energy, particularly a workpiece whose surface is not accessible or only accessible with difficulty, including a workpiece exhibiting a hot surface, it is necessary to receive the acoustic energy in a contact-free manner since liquid coupling means cannot be employed. The question of using gas as a coupling medium could arise, however, the very large difference of acoustic impedance between gas and a solid precludes the use of the latter medium. For ultrasonic testing sound waves are used which are propagated normal as well as at an oblique angle to the workpiece surface. The angle of sound propagation is determined largely by the orientation and location of the defect to be detected. It will be apparent that the directional characteristic of the receiver, that is, its angular sensitivity, must correspond to the angle of propagation of the acoustic wave.
Contact-free receiving means for sound waves having an incidence normal to a surface are known, and the deformation of the surface responsive to sonic energy can be measured, see "Werkstoffprufung mit Ultraschall" (book) Krautkramer, 3rd edition 1975, Springer-Verlag, Berlin/Heidelberg, page 169 and U.S. Pat. No. 4,046,477 of W. Kaule, dated Sept. 6, 1977, entitled "Interferometric Method and Apparatus for Sensing Surface Deformation of a Workpiece Subjected to Acoustic Energy".
However, no means have been developed heretofore to provide the opto-electrical receivers adapted to measure the minute surface deformation of a workpiece responsive to acoustic energy with a directional characteristic.
An important object of the present invention, therefore, is the provision of an optical receiving means used for contact-free reception of ultrasonic waves having a directional characteristic. The directional characteristic optimizes the reception of ultrasonic waves which are propagated in a workpiece at an angle deviating from perpendicular to the workpiece surface.